Heretofore known is a tool changing device secured to a wrist portion of an industrial robot, the device comprising a cylinder-formed main body, a cramp piston reciprocable within the main body, cramp balls displaced by the cramp piston, and an adapter supporting a cutting tool. When the cramp piston is operated, the cramp balls are displaced so as to cramp or release the adapter (refer to, for instance, Japanese Patent Laid-Open No. 186589/1983).
A typical construction of such a conventional device is illustrated in FIGS. 6, 7 and 8. In FIG. 6 showing a longitudinal sectional view, an end surface 110a of a changer main body 110 is secured to a wrist portion of an industrial robot (not shown). When a pressurized fluid is forced into a space 126 within a cylinder portion 112 of the main body 110 through an inlet/outlet port 128, a piston 120 is displaced rightward as viewed in FIG. 6. When the fluid is discharged, the piston 120 is retracted by the resilience force of a spring 122.
A cramp ball holder 116 is secured to the main body 110 by means of screws. Three cramp balls 114 are held by the ball holder 116 freely rotatably. An annular head 118 is further secured to the cramp ball holder 116 and the changer main body 110 by means of balls. Conical surfaces 134 and 136 of different tapers are formed inside of the annular head 118. Numeral 124 designates a positioning pin, and numerals 130 and 142 designate 0 rings that seal the cramp balls 114 and else. Through a hole 146 shown by dotted lines in FIG. 7, a pressurized fluid is introduced into an engaging portion between the main body 110 and a changer adapter 150 so as to control the operation of a hand (not shown) to be secured to a surface 150a of the adapter 150. As shown in FIG. 8, the changer adapter 150 has conical tapered surfaces 154 and 156. When the changer adapter 150 is engaged with the head 118, the conical surfaces 154 and 156 of the adapter 150 are brought into engagement with the conical surfaces 134 and 136 of the head 118, respectively.
Since the changer main body 110 and the changer adapter 150 are constructed as described above, the changer adapter 150 is firstly inserted into the forward opening end of the changer main body 110 with the positioning pin 124 inserted into an axially disposed hole 174 provided in the adapter 150 for aligning the positions of the main body 110 and the changer adapter 150, and engaging the conical surfaces 134 and 138 of the head 118 with the conical surfaces 154 and 156 of the adapter 150, respectively. Then, the piston 120 is advanced forward so as to push the cramp balls 114 onto an engaging surface 152 of the adapter 150, and to cramp hold the adapter 150 within the main body 110.
With the above described construction of the tool changing device, however, at the time of cramping tool, the cramp balls 114 are forced to contact with a straight portion 120a of the piston 120. As a result, the contacting parts of the cramp balls 114, piston 120 and the adapter 150 tend to wear after repetitive contacting and releasing operations, thus creating plays and causing erroneous positioning of the cutting tool. For avoiding the wear, it may be so constructed that the cramp balls contact with a tapered portion 120b of the piston 120 at the time where the adapter 150 is cramped. In this case, however, if the pressure inside of the cylinder is reduced by a trouble of the fluid source or a trouble in the fluid supplying hose, the piston 120 tends to be retracted by an outside force applied to the adapter 150, so that the cramp balls 114 are released from the adapter 150, and the cutting tool is thrown out of the tool changing device, thus causing a dangerous situation.
In a case where a compression spring is provided in the cylinder for preventing the retraction of the piston 120, the operational life of the spring becomes a problem.
An object of the present invention is to provide a tool changing device for a robot of high precision and high reliability, wherein even in a case where the supply of the pressurized fluid is interrupted because of a trouble, any possibility of the tool being thrown away due to an outside force can be substantially eliminated, and the play during the cramped time do not occur regardless of the long-years usage.